The Tyrannosaur Chronicles: The Biology of the Tyrant Dinosaurs

Summary

'Gripping and wonderfully informative' Tom Holland, New Statesman

Adored by children and adults alike, Tyrannosaurus is the most famous dinosaur in the world, one that pops up again and again in pop culture, often battling other beasts such as King Kong, Triceratops or velociraptors in Jurassic Park. But despite the hype, Tyrannosaurus and the other tyrannosaurs are fascinating animals in their own right, and are among the best-studied of all dinosaurs.

Tyrannosaurs started small, but over the course of 100 million years evolved into the giant carnivorous bone-crushers that continue to inspire awe in palaeontologists, screenplay writers, sci-fi novelists and the general public alike. Tyrannosaurus itself was truly impressive; it topped six tons, was more than 12m (40 feet) long, and had the largest head and most powerful bite of any land animal in history.

The Tyrannosaur Chronicles tracks the rise of these dinosaurs, and presents the latest research into their biology, showing off more than just their impressive statistics – tyrannosaurs had feathers and fought and even ate each other. This book presents the science behind this research; it tells the story of the group through their anatomy, ecology and behaviour, exploring how they came to be the dominant terrestrial predators of the Mesozoic and, in more recent times, one of the great icons of biology.

Preface

When writing a science book of close to 90,000 words long, you can be pretty sure of two things. One: it will contain at least a couple, and probably several, pretty bad errors; and two: it will be out of date by the time it is published. The first I am largely resigned to, the second is more important to understand. The rate of discovery and scientific exploration of dinosaurs is accelerating all the time to the point that, on average, a new species is named every week or so; this is in addition to all the other new studies and assessments of behaviour, anatomy, ecology, evolution and the like that will be published in that time.

Since the tyrannosaurs were first recognised as a group of dinosaurs in 1905 with the naming of the genus Tyrannosaurus, a huge number of scientific works have been written about them. A quick search of my own, far from comprehensive digital library finds more than 1,500 papers and books that relate to the subject. One famous paper is nearly 150 pages in length and is primarily a description of a single skeleton. We do know an awful lot about tyrannosaurs, and that knowledge is expanding faster than I have been able to write this book. Already I have had to add to or rewrite sections several times to take account of new ideas, new data and even entire new species that have been described while this book was being composed.

This is not intended as a text book or reference work, so I have skimmed over citing much of the formal scientific literature that forms the basis of our knowledge of the tyrannosaurs. An exhaustive list of papers on tyrannosaurs alone might be longer than this book, so I couldn’t include them all, even if I had wanted to. However, it is important to try and mention key papers, and to show the scientific basis of the ideas and hypotheses laid out over the course of this work. Where appropriate, numbered references to sources are given in the text, and the sources are listed in full at the back of the book. While the reference list is effectively far from complete, as far as possible the information here is backed by at least some scientific studies (a number, of course, are controversial, uncertain or even contradictory), except where I have made it clear that something is based primarily on my own opinions and ideas. Even so, for every paper in the reference list there are perhaps dozens more that explore the ideas expounded upon, and similarly for every fossil illustrated or mentioned, there may be dozens or hundreds more specimens that have been the subject of study or analysis to support an idea.

In this book, I have attempted to steer a middle course covering primarily what I think is the consensus opinion among dinosaur researchers. While minority ideas get a look in, the scope of the book limits the discussion of some areas. I have tried to streamline and simplify often difficult and complex issues, but without overlooking nuance or important contradictions, and to give credit as far as possible given the imposed limitations on space – the intention is to provide a fair and balanced look at exactly what I think the tyrannosaurs were like.

Note From The Illustrator

Scientific illustration of fossils is a curious thing. The goal is to provide the most accurate possible view of extinct organisms, in this case tyrant dinosaurs and their relatives. Naturally, this can’t be a literal view – after all, some of the bones are usually missing, and many of those that survived were distorted by being squashed under tons of rock for tens of millions of years. To put the skeletons together in a literal fashion would render them incomplete and often oddly twisted upon themselves.

Instead, I have endeavoured to provide the most accurate possible portrayal of the skeletons as they were in the living organism. To this end, the first job was to get the proportions right. I have tried to take my own measurements of the original bones where I can, and where expense and distance have made that prohibitive, I have relied on measurements supplied by colleagues. They have all been checked against the vast and ever-growing scientific literature on dinosaurs.

Illustrating dinosaur bones in the proper shapes and proportions is all well and good, but a full skeletal reconstruction requires their arrangement in a manner consistent with the living animal. For this I have relied on detailed observation of specimens, the vast scientific literature on biomechanics and functional morphology, and dissection of extant organisms for comparative purposes.

Wherever possible, I have tried to make the creation of anatomical diagrams a data-driven process, though naturally there are limitations to our state of understanding. Missing data has been filled in from other specimens of the species or their close relatives, and logical anatomical inference was used when hard data was missing (Fig. 1). Future discoveries will undoubtedly require revisions of the interpretations.

Fig. 1 A panoply of Tyrannosaurus skeletons. Few dinosaur fossils are truly complete with every bone known and in good condition, but there is substantial overlap between many and so we can have great confidence in restoring the complete skeleton from these incomplete remains.

Despite any yet-undiscovered errors, every attempt has been made to provide a visual representation of these extinct beasts that matches our current understanding of them. I hope you find them as fascinating as I do.

Scott Hartman

The Game of The Name

Throughout this book there are references to various scientific names of tyrannosaurs, and indeed other dinosaurs – both individual genera and species and also formal evolutionary groups (termed ‘clades’). Some of the terminology and rules for these names and their creation may seem complex and obtuse, but it is important to use them. Scientific terminology is there precisely to provide something that is specific and accurate, and not ambiguous. There’s little point in reinventing the wheel.

Few of the old taxonomic ranks of organisms (Kingdom, Phylum, Class, Order, Family, Genus and Species) are used by modern biologists and palaeontologists. While terms like ‘the dog family’ and ‘Class Aves’ still get bandied about, researchers are increasingly abandoning them since they don’t have clear equivalents between groups. We do still think of groups within groups (so all humans are apes, all apes are primates, all primates are mammals and so on), and the technical names are still used to designate those clades (Homo, Hominidae, Primata, Mammalia), but the ranks are not considered a part of this.

The exceptions are the genus and species names, the traditional scientific or binomial name (often called a Latin name). A few of these at least will be recognisable to many people, for example Homo sapiens, Boa constrictor and, yes, Tyrannosaurus rex. A species is the basic unit of biology, and effectively denotes a group of individuals that are more closely related to each other than to any other individuals (other species). Biologists actually have a hard time defining species due to the bewildering variety of organisms out there and the fact that in the course of evolution species and lineages are constantly changing. The individuals that make up the species Boa constrictor right now, are not those that will be around ten or a hundred years from now, or those that were around a thousand years ago. Species ultimately blur into one another, though of course that’s generally hard to see on the scale of a human lifetime or in the fossil record.

When species are named they are assigned to a genus, and the two names are used together to correctly identify an organism, which is important as there may be multiple species within a single genus. In the case of dinosaurs almost all genera contain just a single species, and as a result researchers (and the public) tend to refer primarily to the generic name – hence Triceratops and Diplodocus, but not often Triceratops horridus or Diplodocus carnegiei, or for that matter Diplodocus longus. A reduced form of the name is often used, which only includes the initial of the genus – thus Tyrannosaurus rex would become T. rex. Genus and species names are both italicised, and the generic name but not the specific one gets a capital. In the case of the tyrannosaurs, every genus named except Alioramus currently has only a single species in it, so for simplicity only the genus name is generally used as it is unambiguous (for example Tyrannosaurus rather than Tyrannosaurus rex).

The definition of a species you might well recognise (a group of organisms capable of breeding with one another) certainly is one definition of a species in use, but it’s not much use for asexual bacteria, or for that matter extinct animals that are known only from fossils, so plenty of other definitions are also in common use. In the case of fossils, palaeontologists use a ‘morphological species concept’ – in short, they ask whether the organisms have a series of consistent anatomical differences that are likely to have been reflected in the living species. So, for example, when identifying different fossils, size is not a good basis for comparison (lots of species have markedly differently sized individuals in their ranks), nor are subtle differences such as having one more or one fewer teeth. However, a major difference like having much longer legs, or three fingers instead of four, or a crest on the skull, can be more convincing. Even so, there can be disagreements about whether or not two skeletons are effectively ‘different enough’ to warrant being named as separate species, or separate genera, and the criteria tend to be different between groups, depending on the data available and to a degree the researchers doing the work.

Who is related to whom

In the last 20 years or so, a great deal of work has gone into working out the relationships between species, including for dinosaurs. Essentially the various characters of an organism are tallied and compared with other species. Those with the most features in common are considered the nearest relatives of one another, since they have diverged least from their common ancestor. In the case of the tyrannosaurs, there are five main groups to deal with – the tyrannosauroids and tyrannosaurids, and then the protoceratosaurids, albertosaurines and tyrannosaurines (see Fig. 2). Some groups lie within others, so all tyrannosaurines and albertosaurines are also tyrannosaurids, and all tyrannosaurids are tyrannosauroids. We can also use these terms to split off the non-included genera, for example by talking about non-tyrannosaurine tyrannosaurids. This system appears a little unwieldy at first, but it is relatively easy to get used to.

Fig. 2 A very simple phylogeny of the major tyrannosaur clades.

Throughout this book the terms ‘tyrannosaur’ and also ‘tyrant dinosaur’ are used to be essentially synonymous with ‘tyrannosauroid’, encompassing all the animals shown in Fig. 2. These are not formal scientific terms, but they are convenient and I think appropriate. Palaeontologists use the term ‘non-avian dinosaurs’ because birds are really dinosaurs – here, however, to help readability, the name ‘dinosaur’ encompasses only the ‘traditional’ dinosaurs and excludes the birds.

There’s a time and place for everything

It is important to place the tyrannosaurs in the correct temporal and geographic contexts if we are to understand them fully. The world they occupied (one could even say ‘worlds’, given the times and changes involved) was very different from the one we would recognise today. The continents lay in different locations, the climate was different, the species around them (competitors, prey, plants, parasites and so on) were different, and these factors all influenced how the tyrannosaurs lived, evolved and died.

The surface of our planet is forever changing. Although a look out of your window may not reveal much difference from day to day, the twin effects of erosion and deposition are ever at work: taking material from one place and depositing it in another. Occasional dramatic events like floods, volcanic eruptions and rock slides can move thousands of tons of material in minutes, but mostly such things happen too slowly to be seen or appreciated on a human timescale – given enough time, however, the mountains really will crumble to the sea.

The movements of the continents are even slower. In the course of occasional ‘exciting’ events like earthquakes, continental plates may move a number of metres in seconds, although the rate is more normally a few millimetres over a year. Again, however, these events are hard to appreciate over a lifetime that is rarely as long as a century – yet with the dinosaurs we are often looking at continental changes that took place over millions or even tens of millions of years.

Importantly, factors like the positions of the continents influence local climate and weather, and also limit or allow movement between land masses for various species. It should also be borne in mind that due to the movements of the continents, many fossils may have moved from their locations in the past. In the Jurassic it would have been relatively easy to walk from South America to Australia via Antarctica, not only because the climate was much warmer then, but also because those land masses were joined together.

Some of the time spans are colossal and hard to imagine. The first dinosaurs arose around 240 million years ago (mya) in a period called the Late Triassic. The first tyrannosaurs did not come onto the scene for quite some time, with the earliest that we know of appearing in the Middle Jurassic period around 80 million years later (160 mya). The last of the non-avian dinosaurs, including the last of the tyrannosaurs, went extinct at the end of the Late Cretaceous period around 66 mya, so tyrannosaurs of one form or another were around for the best part of 100 million years.

Starting in the Late Triassic through to the End Cretaceous, the continents have shifted from something close to a single great land mass, to a layout not too far from the present one. The climate has (overall) cooled since then – the Triassic lacked ice at the poles – and the life that occupied these lands has changed dramatically, too. A visitor to the Late Triassic would recognise a number of plants and animals as being close to those still alive today. Early mammals that were rather rat- or possum-like would be scurrying around, there would be lizards, tortoises and crocodiles, and plenty of recognisable insects, spiders, millipedes and other invertebrates, while ferns, cycads and horsetails would have been common. Non-dinosaurian ancient reptiles such as pterosaurs in the air and icthyosaurs in the sea would also abound from the Triassic onwards, while in the Jurassic and Cretaceous the flora and fauna would become still more familiar, with lineages such as birds, snakes, grasses, magnolias and monkey puzzle trees appearing and becoming common.

As we will see, the place of the tyrannosaurs within these ecosystems changed dramatically over the 100 million years, from small and probably rare members of communities, to by far the largest carnivores on the continents they occupied, with the capacity to tackle almost any species around. The changing environment, and the changes to other species around the tyrannosaurs, would have influenced their evolution (as the tyrannosaurs in turn would have influenced those around them). The separation in time and place between some animals (the time difference between the respective appearances of Guanlong and Tyrannosaurus, is greater by a fair margin than the time between the last of the Tyrannosaurus and you reading these words) makes it important to recognise the sheer scale of the ancient past and how this may have shaped long-dead lineages.

A Brief Primer on Tyrannosaur Bony Anatomy

I have limited the amount of anatomical detail and technical terms in this book. People often dismiss technical terms and names as ‘jargon’, but the truth is that a good technical term can be exact and specific, and avoid confusion and long-winded descriptions. ‘Arctometatarsus’ is a tongue twister of a word when you first encounter it, but it’s a lot easier to refer to than ‘that special condition when the middle long bone in the foot is pinched at the upper end and flares out at the base’ every few lines when discussing its evolution and anatomy. There are therefore some anatomical terms in the text, and an introduction to them follows. Although every part of the skeleton has a name and is often associated with still more terminology relevant to its size, shape, orientation and which other parts it links to, only the pieces that most often come up in the discussion of the tyrannosaurs are included here. Names in bold below are marked on the illustrations.

Here the head of the legendary Tyrannosaurus is pictured conveniently side on, or more formally, in left lateral view (Fig. 3a). The skull as a term covers all the bones that make up the head, but can be split into the fundamental units of the mandible (lower jaws) and cranium (everything else). Key bones in the skull are those that hold the teeth: at the front of the cranium is the premaxilla, and behind that the maxilla (one on each side), while the front of each side of the mandible is a dentary. The nasals lie behind and between the maxillae and help bound the nares (the openings for the nostrils in the skull). Behind the naris on each side is an antorbital fenestra, then the orbit (the eye socket) itself. The roof of the mouth (not shown) is termed the palate. Each tooth has a root that sits in the jaw, and a crown that is exposed. The ‘edge’ of the tooth is a carina made up of tiny serrations called denticles.

Fig. 3a The major bones and features of the skull of a tyrannosaur, here based on Tyrannosaurus.

Moving on to a whole Tyrannosaurus (again in left lateral view), it can be seen that tyrannosaurs fit with anatomical orthodoxy, with the skull being located at the front of the animal (Fig. 3b). Moving on to the axial skeleton (the backbone and associated bits), there is the atlas, that is the first vertebra of the neck (though it’s hidden in the figure behind the back of the skull). Behind the atlas are the rest of the cervical vertebrae (neck) and their associated cervical ribs. Then comes the actual back part of the backbone, the dorsal vertebrae and their dorsal ribs. Lying along the belly is a series of fine bony rods occasionally called belly ribs, but better named gastralia. Posterior to the dorsals are the sacral vertebrae, which are fused together to compose the sacrum and make up the major part of the pelvis. After this comes the tail, made up of caudal vertebrae, and below them the chevrons (also called haemal arches in mammals).

Finally there is the appendicular skeleton, that is the limbs and their supporting girdles. Upfront are the shoulder elements of the scapula and coracoid, and between them the furcula and sternum, then the humerus (the upper arm bone), then the lower elements of the radius and ulna, and the wrist (carpals), the hand (metacarpals), and the bones of the fingers (phalanges), terminating with the unguals (the bony claws). The pelvis supports the hindlimb and consists of three pieces on each side: the ilium at the top (hiding the sacrum to which it is fused), the ischium to the rear and the pubis to the front. The legs each consist of a femur first, then a large tibia and smaller fibula, then the tarsals (ankle bones), then the metatarsals, followed again by phalanges and unguals.

Fig. 3b The major bones of the skeleton of a tyrannosaur, here based on Tyrannosaurus.

That is the skeleton in brief. In all there are a couple of hundred bones in a single individual tyrannosaur (and the best part of 100 teeth sticking out of the jaws of most) with the majority occurring in pairs (left and right arms, left and right ilia, left and right maxillae and so on), except the vertebrae and the furcula (though this is actually a pair of bones fused together: the clavicles or collarbones). Most fossil specimens are only a small fraction of this total count, and even skeletons described as being ‘complete’ may have a good number of bones missing (though of course if you have a left arm, you know what the right one looked like and so on).

There are some features unique to tyrannosaurs compared with other dinosaurs (like the hugely expanded free end of the pubis – termed the ‘boot’). However, the overall plan of the skeleton is typically dinosaurian, and is pretty typical of most tetrapods (the group that includes all amphibians, reptiles, birds and mammals). Those familiar with human anatomy or that of other animals, like mammals and birds, will recognise a lot of these names as a result. The names are mostly shared because in an evolutionary sense, the bones are the same (although because anatomical terms were developed separately by various people at various times, mammal, bird and reptile anatomists can use different terms for the same bones). The skull has most of the same bones in the same places, the backbone is the same, there are four limbs, each with one bone, then a pair, then some smaller ones to form a joint, then fingers or toes. The evolutionary pattern of the tetrapod skeleton was set early on and remains pretty similar in almost all animals, with few gains along the way (for example, many birds have long necks with numerous extra vertebrae), and a fair number of losses (we barely have a tail, and snakes and whales have all but lost their legs). What dictates the difference between various animals is the shape and structure of these elements, and how they link to muscles, blood vessels, lungs and other anatomical features to build different-looking (and acting) species.

Part one

Introduction

CHAPTER ONE

INTRODUCING THE DINOSAURS

Around 160 million years or so ago, probably somewhere in the northern hemisphere, a new lineage of carnivorous dinosaurs began to separate from its relatives. They were probably not that different from their contemporaries to look at – after all, evolution doesn’t happen overnight. These animals were pretty small, and didn’t have especially big heads, long arms or short tails. They were just carnivorous bipedal (two-legged) dinosaurs out looking for a meal, taking what meat they could find and staying out of the way of the bigger beasts that populated the landscape.

However, given time, this new line of animals would come to include the largest terrestrial carnivores of all time, with giant heads, huge teeth, tiny arms and highly modified feet. The biggest individuals were perhaps around 14 metres in length and weighed over 8 tonne. They would be the subject of more research than any other lineage of dinosaurs (aside from the birds), and they would become icons of evolution, and of the dinosaurs as a whole. These were the tyrannosaurs – Tyrannosaurus and its ancestors and relatives – a group of dinosaurs that lived around the world for some 100 million years, represented by at least 25 known species. Perhaps no scientific name is as well known as Tyrannosaurus rex (and I am certain none is incorrectly written so often), and the animal that bears that name is easily the most popular and best-known dinosaur with the general public. Jurassic Park and King Kong would not have been the same without it, and visitor numbers increase every time a museum displays a new skeleton or animatronic model. It is hard to put a price on fame, but a skeleton of the largest and most complete individual T. rex known sold for millions of dollars in 1997 (Fig. 4).

Fig. 4 A restored skeleton of Tyrannosaurus rex based primarily on the famous and large specimen known as ‘Sue’.

The tyrannosaurs are endlessly charismatic: their size, appearance and reputation precede them wherever they go, and there is almost no media article or report on dinosaurs that doesn’t mention them or relate any new find to tyrannosaurs in some way (‘as large as’, ‘a relative of’, ‘lived 75 million years before’, and on and on). There is also a huge number of tropes and memes about the animals that follow them around, and endless repetition does not make them right. Tyrannosaurs were not pure scavengers; they didn’t spend their lives battling adult Triceratops, they did not have poor eyesight, they could not run at 50 km/h, females were not bigger than males, and so on. Unfortunately, the hype and hyperbole overshadows a fascinating evolutionary story: how and why did they get so large, why the tiny arms, why the giant heads, how did they act? We know so much about the animals in this group – their anatomy, evolution, behaviour and general biology – but it’s almost impossible to say very much over the chorus of statements about how cool they are or questions as to whether they would win in a fight with Spinosaurus.

Exploring the wealth of scientific data and analyses will take in bones, skin, feathers, footprints, scans of tyrannosaur skulls, possible evidence of cartilage surviving for 66 million years, their muscle attachments, their injuries and their bite power, and even studies of entire ecosystems. There are skeletons and teeth from Australia through Mongolia, from Britain, Canada and Brazil; the story of the tyrannosaurs covers more than 100 million years of palaeontological history, and over a century of scientific research.

Awaking the lost world

The story of the dinosaurs begins in Britain in the early to middle part of the nineteenth century. In 1824, the then Reverend (and later Canon) William Buckland, a British naturalist based at the University of Oxford, named the first ever dinosaur: Megalosaurus. Known for some time from little more than a

I received this book free from NetGalley and Bloomsbury Sigma in exchange for an honest and fair review. Thank you! As paleontology books go this is one of the most informative and entertaining books I've read in some time. This book is full of facts and diagrams about the hunting, mating and social habits of the biggest predator to have existed on earth. The book is highly accessible to the novice and dinosaur enthusiast and doesn’t talk down or dilute the technical aspects of paleontology. The attention to detail is impressive and the range of topics wide ranging. The book begins with a description of the anatomy and includes diagrammatic hypothesis about the history of the evolutionary relationships of Tyrannosaurs and other organisms. I found this gave me a foundation in the terms used, so that in later chapters I could follow what the author was conveying. The chapters on possible Tyrannosaur behavior, ecology and future of paleontology were extremely thought provoking.The author has a terrific writing style and doesn’t bombard the reader with unnecessary jargon but remains technically sufficient to deliver an in-depth, precise book without being boring.A must have for any dinosaur fan.